1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus, especially useful for an open-type MRI apparatus that has an open magnet gantry.
2. Description of the Related Art
Earlier open-type MRI systems include a vertical field type MRI system (MRI system with open access to patient image volume), featuring open access to the patient image volume, as disclosed for example in U.S. Pat. No. 4,829,252.
In an MRI system featuring good open access, such as the above-noted vertical field type of open MRI system or a tubular type MRI system of the past with a short axis and large diameter, the patient couch is moved front and back and to the left and right at a uniform height with respect to the patient transport axis when acquiring images. In this case, in the open MRI system, there are not good gradient magnetic field linearity and static magnetic field uniformity in comparison with the earlier tubular type MRI system. However, because there is good magnetic field uniformity at the center of the gradient magnetic field and static magnetic field, if images of the region of treatment of the patient are acquired at the center position of the magnetic field, high-quality images of the region of treatment are obtained. In this case, there is needed a complicated process in which a mark from a positioning projector is set on a specific point of the patient or T/R coil before moving the patient into the gantry as a preparation of setting the patient on the center of the magnetic field. This is to inform the MRI main unit of distance information between the mark and the center of the magnetic field in a front/rear direction and a right/left direction with respect to the patient transport axis.
When operating the above-noted type of open MRI as an interventional apparatus, for example, the region (location) of treatment is often not large, but limited to a specific area. In the case, even if the patient couch is moved within a horizontal plane with respect to the patient transport axis at a uniform height, it was either extremely difficult or impossible to position the region of treatment at the center of the gradient magnetic field and static magnetic field in three dimensions.
For this reason, the images obtained from such MRI systems in the past exhibited a great deal of image distortion, image non-uniformities, and fat artifacts, making it difficult to use the images in treatment.
Accordingly, it is an object of the present invention to provide a magnetic resonance imaging apparatus which enables quick positioning of the region of treatment or diagnosis at the center of the gradient magnetic field and static magnetic field, and enables the acquisition of highly precise, high-quality images, with reduced image distortion, non-uniformities, and fat artifacts.
In order to achieve the above-noted object, a magnetic resonance imaging apparatus according to the present invention comprises:
a static magnetic field generator for generating a static field;
a gradient magnetic field generator for generating a gradient magnetic field that is superimposed on the static magnetic field;
a radio-frequency magnetic field pulse transmitting/receiving unit, which applies a radio-frequency pulse to a region of interest of the patient that is located within the static magnetic field, and which also receives a magnetic resonance signal that is generated from the patient;
a patient couch, which enables movement of the patient;
a position information establishing apparatus which establishes position information of the region of interest of the patient; and
a patient couch controller for moving the patient couch, based on the region of interest position information, so that the region of interest is positioned either at the center of the static magnetic field, or at the center of the gradient magnetic field.
According to the present invention, because a patient couch controlling means causes the movement of the movable patient couch, so as to position the region of interest of the patient at the center of either the static magnetic field or the gradient magnetic field, it is possible to obtain precise, high-quality images, with reduced image distortion, non-uniformities, and fat artifacts.
Another aspect of the present invention is a method for performing magnetic resonance imaging diagnosis, this method comprising the steps of:
placing the patient onto a patient couch that is disposed within a static magnetic field and a gradient magnetic field;
moving the patient couch approximately, based on a signal from a position detector, so that the region of interest of the patient approximately coincides with the center of the static magnetic field or the center of the gradient magnetic field;
applying a radio-frequency pulse to the region of interest of the patient, and receiving a magnetic resonance signal that is generated from the patient;
reproducing a plurality of images of the patient, based on the magnetic resonance signal;
selecting an image that includes the region of interest from the plurality of images of the patient; and
moving the patient couch, based on the selected image, so that the region of interest of the patient coincides precisely with the center of the static magnetic field or the center of the gradient magnetic field.
The above-noted method of diagnosis according to the present invention provides the same effect as the earlier described magnetic resonance imaging apparatus.